Get rid of duplicate definition of __has_attribute.

Get rid of duplicate definition of __has_attribute.

compiler-tests.h already checks whether __has_attribute and, if not,
defines it as a macro that always returns 0 (so that we use the "what
version of the compiler is this?" fallback checks for a given
attribute).  We include compiler-tests.h before we define
__has_attribute ourselves, so we don't need to do it ourselves.