Bug 1580931 - remove libc++ gcc 4.9 workarounds from toolchain.configure; r=nalexander

Getting rid of unneeded code and having fewer global defines are both
worthy things.

Differential Revision: https://phabricator.services.mozilla.com/D45726

--HG--
extra : moz-landing-system : lando

build/moz.configure/toolchain.configure

@@ -1386,11 +1386,7 @@ def libcxx_override_visibility(c_compiler, target):
 
 
 set_define('_LIBCPP_INLINE_VISIBILITY', libcxx_override_visibility.empty)
-set_define('_LIBCPP_INLINE_VISIBILITY_EXCEPT_GCC49',
-           libcxx_override_visibility.empty)
 set_define('_LIBCPP_ALWAYS_INLINE', libcxx_override_visibility.empty)
-set_define('_LIBCPP_ALWAYS_INLINE_EXCEPT_GCC49',
-           libcxx_override_visibility.empty)
 
 set_define('_LIBCPP_HIDE_FROM_ABI', libcxx_override_visibility.hide_from_abi)