Removed unneeded functions from lbuxton-simp.c

Remove unneeded functions - include all type-specific callbacks and
one create_group function and one remove group function. The callbacks
are replaced by one callback with a switch that handles all data
types. Renamed buxtond_remove_group2 to buxtond_remove_group for
simplicity. All changes are reflected in buxton-simp.h, lbuxton-simp.sym,
and lbuxton-simp_test.c
TODO: replace man page sample code with updated function name
Updated man pages with new function name buxtond_remove_group()