ccache: drop CCACHE_COMPILERCHECK

ccache: drop CCACHE_COMPILERCHECK

This variable controls how ccache determines if the cache is valid or not
for a given compiler.

By default, ccache looks at the mtime of the compiler binary. This should
be sufficient for our purposes as the mtime is preserved from sstate
and will only be changed on rebuilds if the binary is actually different.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>