perl: remove references to Memory::Usage module for perl module tests

perl: remove references to Memory::Usage module for perl module tests

The perl memory test was removed with commits

https://git.savannah.gnu.org/cgit/pspp.git/commit/?id=20934a68593edbd52577f1ba0ea93c74eb018af5

and

https://git.savannah.gnu.org/cgit/pspp.git/commit/?id=05297509d237dc99ba7504ccf260e995586bc3b4

This commit removes the remaining references to the perl memory test.
The configure step still asked for optional Memory::Usage perl module
which is not used without the perl memory test.