Remove an obsolete documentation of (make binary-static)

... which no longer works after #932.

This does not add documentation for the current static build approach,
nor does it add any other place where DISABLE_CGO is documented;
both are not tested by CI, and discouraged due to bad integration
with the rest of the system.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

Author: mtrmac

install.md

@@ -130,12 +130,6 @@ Building in a container is simpler, but more restrictive:
 $ make binary # Or (make all) to also build documentation, see below.
 ```
 
-To build a pure-Go static binary (disables devicemapper, btrfs, and gpgme):
-
-```bash
-$ make binary-static DISABLE_CGO=1
-```
-
 ### Building documentation
 
 To build the manual you will need go-md2man.