[Feature #19442] Remove USE_RINCGC flag

Ruby doesn't compile when this is set to 0. Let's remove it.

.github/workflows/compilers.yml

@@ -165,7 +165,6 @@ jobs:
           - { name: USE_FLONUM=0,                   env: { cppflags: '-DUSE_FLONUM=0' } }
 #         - { name: USE_GC_MALLOC_OBJ_INFO_DETAILS, env: { cppflags: '-DUSE_GC_MALLOC_OBJ_INFO_DETAILS' } }
           - { name: USE_LAZY_LOAD,                  env: { cppflags: '-DUSE_LAZY_LOAD' } }
-#         - { name: USE_RINCGC=0,                   env: { cppflags: '-DUSE_RINCGC=0' } }
 #         - { name: USE_SYMBOL_GC=0,                env: { cppflags: '-DUSE_SYMBOL_GC=0' } }
 #         - { name: USE_THREAD_CACHE=0,             env: { cppflags: '-DUSE_THREAD_CACHE=0' } }
 #         - { name: USE_TRANSIENT_HEAP=0,           env: { cppflags: '-DUSE_TRANSIENT_HEAP=0' } }

gc.c

@@ -470,7 +470,7 @@ int ruby_rgengc_debug;
 #define GC_PROFILE_DETAIL_MEMORY 0
 #endif
 #ifndef GC_ENABLE_INCREMENTAL_MARK
-#define GC_ENABLE_INCREMENTAL_MARK USE_RINCGC
+#define GC_ENABLE_INCREMENTAL_MARK 1
 #endif
 #ifndef GC_ENABLE_LAZY_SWEEP
 #define GC_ENABLE_LAZY_SWEEP   1