remove base_type_eq

The last uses have been removed, and base_type_eq has been deprecated since
2019; a previous attempt to remove it (#4056) has failed.

Author: kroening

src/util/Makefile

@@ -1,7 +1,6 @@
 SRC = arith_tools.cpp \
       array_element_from_pointer.cpp \
       array_name.cpp \
-      base_type.cpp \
       bitvector_expr.cpp \
       bitvector_types.cpp \
       bv_arithmetic.cpp \