sys-devel/binutils-config: drop runtime linker switch selection

sys-devel/binutils-config: drop runtime linker switch selection

Before this change there were 3 ways to switch default linker:
1. via LDFLAGS=-fuse-ld=gold LD=${CHOST}-ld.gold
2. via binutils' USE flags: USE="default-gold gold"
3. via *FLAGS=-B/path-to-own-tools
4. via 'binutils-config --linker'

This change drops [3.] entirely in favour of [1.] and [2.].
[1.] should be preferred.

Reported-by: Andrzej Korwin-Mikke
Closes: https://bugs.gentoo.org/658054
Package-Manager: Portage-2.3.84, Repoman-2.3.20
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>