Remove the version check for the dynamic build configuration

Upstream is very stale at the moment and not bumping this regularly.
However, at the time of writing git2go still works fine with the most
recent libgit2 version. Therefore, just remove the version check for
now until upstream work is resumed.

Build_system_dynamic.go

@@ -8,8 +8,5 @@ package git
 #cgo CFLAGS: -DLIBGIT2_DYNAMIC
 #include <git2.h>
 
-#if LIBGIT2_VER_MAJOR != 1 || LIBGIT2_VER_MINOR < 5 || LIBGIT2_VER_MINOR > 5
-# error "Invalid libgit2 version; this git2go supports libgit2 between v1.5.0 and v1.5.0"
-#endif
 */
 import "C"