Rename go version dependent sources using idiomatic naming

The Go toolchain has special support for build tags for Go versions in
source filenames.
https://golang.org/pkg/go/build/#hdr-Build_Constraints
This makes build tags redundant, so they are removed.

Author: dolmen

benchmark_go18_test.go benchmark_go1.8_test.gobenchmark_go18_test.go renamed to benchmark_go1.8_test.go

@@ -6,8 +6,6 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-// +build go1.8
-
 package mysql
 
 import (

connection_go18.go connection_go1.8.goconnection_go18.go renamed to connection_go1.8.go

@@ -6,8 +6,6 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-// +build go1.8
-
 package mysql
 
 import (

connection_go18_test.go connection_go1.8_test.goconnection_go18_test.go renamed to connection_go1.8_test.go

@@ -6,8 +6,6 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-// +build go1.8
-
 package mysql
 
 import (

driver_go18_test.go driver_go1.8_test.godriver_go18_test.go renamed to driver_go1.8_test.go

@@ -6,8 +6,6 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-// +build go1.8
-
 package mysql
 
 import (

utils_go17.go utils_go1.7.goutils_go17.go renamed to utils_go1.7.go

@@ -6,8 +6,6 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-// +build go1.8
-
 package mysql
 
 import (

utils_go18.go utils_go1.8.goutils_go18.go renamed to utils_go1.8.go

@@ -6,8 +6,6 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-// +build go1.8
-
 package mysql
 
 import (