configure: remove pod2man from the config list

The configure has the --disable-manpages option for this purpose, and
--disable-pod2man is currently ignored due to that. This is also
consistent with the other documentation options.

configure

@@ -1702,7 +1702,6 @@ CONFIG_LIST="
     memory_poisoning
     neon_clobber_test
     pic
-    pod2man
     raise_major
     thumb
     valgrind_backtrace