dev-libs/gmp: drop USE=pgo from stable ebuild, bug #658688

dev-libs/gmp: drop USE=pgo from stable ebuild, bug #658688

USE=pgo generates optimal constants when running 'tuneup' benchmark
locally. If benchmark does not succeed default parameters are used.
Else benchmark's output is used to tune gmp behaviour.

Unfortunately at least on x86 some primitives like
__mpn_sqr_basecase generate invalid assembly code at fail tests.

In bug #650558 we found out that 'tuneup' is not very well maintained
upstream. Let's dropp support for USE=pgo until it gets better.

Reported-by: Robert Gill
Closes: https://bugs.gentoo.org/658688
Bug: https://bugs.gentoo.org/650558
Package-Manager: Portage-2.3.59, Repoman-2.3.12
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>