Remove unused ENABLE_GC_FRIENDLY_DEFAULT and its build option
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.