ENABLE_GC_FRIENDLY_DEFAULT was supposed to set the default for the gc friendliness
while still allowing to force enable it at runtime with G_DEBUG=gc-friendly.

With commit 943a18b5 (6 years ago) things were changed to always set it
according to the content of G_DEBUG in glib_init(), making the default unused.

Since nobody complained since then just remove the macro and the build option.