lang/python39: Clean up unnecessary PLIST_SUB

- Remove XY: Replace .cpython-%%XY%% with %%PYTHON_EXT_SUFFIX%%
- Remove XYDOT:
  - Replace lib/python%%XYDOT%% with %%PYTHON_LIBDIR%%
  - Replace %%XYDOT%% with %%PYTHON_VER%%
- Rename XYZDOT to DISTVERSION

lang/python39/Makefile

@@ -41,9 +41,7 @@ SUB_FILES=		pkg-message
 SUB_LIST=		PYTHON_SUFFIX=${PYTHON_SUFFIX}
 
 PLIST_SUB=		ABI=${ABIFLAGS} \
-			XY=${PYTHON_SUFFIX} \
-			XYDOT=${PYTHON_VER} \
-			XYZDOT=${DISTVERSION} \
+			DISTVERSION=${DISTVERSION} \
 			OSMAJOR=${OSVERSION:C/([0-9]*)[0-9]{5}/\1/}		# For plat-freebsd* in pkg-plist. https://bugs.python.org/issue19554
 
 OPTIONS_DEFINE=		DEBUG IPV6 LIBMPDEC LTO NLS PYMALLOC