gh-115119: Remove superfluous TEST_COVERAGE private macro from _decimal module (GH-149756)

It was previously shared with `libmpdec`, which is no longer vendored.

Author: skirpichev

Modules/_decimal/_decimal.c

@@ -188,7 +188,7 @@ find_state_ternary(PyObject *left, PyObject *right, PyObject *modulus)
  *    sizeof(size_t) == sizeof(mpd_uint_t) == sizeof(mpd_ssize_t)
  */
 
-#ifdef TEST_COVERAGE
+#ifdef Py_DEBUG
   #undef Py_LOCAL_INLINE
   #define Py_LOCAL_INLINE Py_LOCAL
 #endif

configure

@@ -4398,11 +4398,6 @@ PKG_CHECK_MODULES([LIBMPDEC], [libmpdec >= 2.5.0], [have_mpdec=yes], [
   ])
 ])
 
-# Disable forced inlining in debug builds, see GH-94847
-AS_VAR_IF(
-  [with_pydebug], [yes],
-  [AS_VAR_APPEND([LIBMPDEC_CFLAGS], [" -DTEST_COVERAGE"])])
-
 # Check whether _decimal should use a coroutine-local or thread-local context
 AC_MSG_CHECKING([for --with-decimal-contextvar])
 AC_ARG_WITH(