Revert "build: target OSX 10.5 when building on darwin"

This reverts commit b6d6a54f80,
which fixed #3072, so we'll have to figure out another way
to make that work.

common.gypi

@@ -171,7 +171,6 @@
           'GCC_VERSION': '4.2',
           'GCC_WARN_ABOUT_MISSING_NEWLINE': 'YES',  # -Wnewline-eof
           'PREBINDING': 'NO',                       # No -Wl,-prebind
-          'MACOSX_DEPLOYMENT_TARGET': '10.5',
           'USE_HEADERMAP': 'NO',
           'OTHER_CFLAGS': [
             '-fno-strict-aliasing',