Remove dead C++ wrappers left over from corehost C conversion

[elinor-fung]

Following the apphost/nethost conversions to C, a handful of thin C++ wrappers in hostmisc (utils and pal) are no longer referenced by anything. This removes that dead code.

Note

This PR description was generated with assistance from GitHub Copilot.

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[Copilot]

Pull request overview

This PR removes unused C++ wrapper APIs in src/native/corehost/hostmisc (utils/pal) that are no longer referenced after the corehost-side C conversions, reducing dead code without intended behavioral impact.

Changes:

• Remove unused DOTNET_ROOT environment wrapper helpers from hostmisc/utils.{h,cpp}.
• Remove unused pal::get_timestamp and pal::is_directory C++ wrappers from hostmisc/pal.{h,unix.cpp,windows.cpp}.
• Keep the remaining C and C++ interop surfaces (e.g., pal_* C APIs and other pal:: helpers) unchanged.

Show a summary per file

File	Description
src/native/corehost/hostmisc/utils.h	Removes unused C++ wrapper declarations for DOTNET_ROOT env var helpers and to_upper.
src/native/corehost/hostmisc/utils.cpp	Removes unused wrapper implementations for DOTNET_ROOT env var helpers and to_upper.
src/native/corehost/hostmisc/pal.h	Removes unused C++ wrapper declarations for get_timestamp and is_directory.
src/native/corehost/hostmisc/pal.windows.cpp	Removes unused Windows implementation of pal::get_timestamp and pal::is_directory.
src/native/corehost/hostmisc/pal.unix.cpp	Removes unused Unix implementation of pal::get_timestamp and pal::is_directory.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 0

[am11]

nit: we can use the minipal version that's generated by ICU raw data (covers the 16-bit range, or 2¹⁶ values). It is much more reliable for international text than ASCII-only tolower:

Suggested change

	[](pal::char_t c) { return static_cast<pal::char_t>(minipal_tolower_invariant(static_cast<CHAR16_T>(c))); });

(remember to add #include <minipal/strings.h>; obj is already linked into corehost)

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (1)

src/native/corehost/hostmisc/utils.cpp:423

• to_lower now lowercases each pal::char_t by calling minipal_tolower_invariant on a CHAR16_T-cast value. On Unix, pal::char_t is char and strings are UTF-8 bytes; if char is unsigned on a given platform, bytes in the range 0x80-0xFF can be interpreted as U+00xx and get Unicode-lowercased, potentially corrupting UTF-8 data. This should only apply Unicode/UTF-16 lowercasing on Windows (where pal::char_t is UTF-16), and use an ASCII-only invariant fold on Unix to avoid mutating non-ASCII bytes.

pal::string_t to_lower(const pal::char_t* in) {
    pal::string_t ret = in;
    std::transform(ret.begin(), ret.end(), ret.begin(),
        [](pal::char_t c) { return static_cast<pal::char_t>(minipal_tolower_invariant(static_cast<CHAR16_T>(c))); });
    return ret;

• Files reviewed: 5/5 changed files
• Comments generated: 0 new

[am11]

LGTM. GCC failure is unrelated #131520.