devel/linux-*-devtools: remove GMP

With GMP in linux_base-* remove its occurrence in the devtools port.

Spotted by:	antoine
Differential Revision:	https://reviews.freebsd.org/D1256
Approved by:	swills (mentor)


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@373783 35697150-7ecd-e111-bb59-0022644237b5