debian/control: Drop libgmp3-dev, gmp4 provides it in :target

Recent versions of GMP are "GPL-2 or LGPL-3+" so we cannot use them in :target because picking GPL-2 would make anything up in the stack (like GnuTLS) unusable from proprietary applications, while picking LGPL-3 goes against our licensing policies.

The current (nasty) solution is to have a very old gmp4 package in :target to produce libgmp3-dev, and have the very old GnuTLS in :target depend on it, while libgmp-dev comes from this package in :development.

This means that this package should not provide libgmp3-dev or APT will get very confused when trying to resolve the build dependencies while building GnuTLS in :target: since the build environment includes both :target and :development.

Unfortunately the recent update in commit 7010f073 lost the downstream change dropping libgmp3-dev, so let's drop it again.