cleanup: remove LEGAL and (support in) related tools.

LEGAL is badly maintained, LEGALlint notices quite a number of ports
which mismatch their line in LEGAL. The port itself is always leading.

• math/giacxcas: restrict RESTRICTED_FILES to giac_* only

Differential Revision: https://reviews.freebsd.org/D30061

Reviewed by: portmgr (mat), dch, eadler, marcus