Simplify lisp.h in minor ways that should not affect code.

Simplify lisp.h in minor ways that should not affect code.

* lisp.h (USE_2_TAGS_FOR_INTS): Remove, as it was always defined.
(LISP_INT_TAG, case_Lisp_Int, LISP_STRING_TAG, LISP_INT_TAG_P)
(LISP_INT1_TAG, enum Lisp_Type, XINT, XUINT, make_number):
Simplify under the assumption that USE_2_TAGS_FOR_INTS is defined.
(INTTYPEBITS): New macro, for clarity.
(INTMASK, MOST_POSITIVE_FIXNUM): Use it.
(LISP_INT1_TAG, LISP_STRING_TAG, LISP_INT_TAG_P): Simplify
now that USE_LSB_TAG is always defined.
(TYPEMASK, XINT) [USE_LSB_TAG]: Remove unnecessary cast.
(make_number) [!USE_LSB_TAG]: Use INTMASK; that's simpler.