Remove LINKABLE from COMPILE_TIME_SETTINGS. NFC

Maybe one day in the part in needed to be here becuase SIDE_MODULE
and MAIN_MODULE are both compile-time settings (since they imply
PIC).  But the automatic setting of LINKABLE, based on MAIN_MODULE
now happens only in link.py.

Author: sbc100

tools/settings.py

@@ -86,7 +86,6 @@
     'MAIN_MODULE',
     'SIDE_MODULE',
     'RELOCATABLE',
-    'LINKABLE',
     'STRICT',
     'EMSCRIPTEN_TRACING',
     'PTHREADS',