rtld: don't add extraneous -L directory when MK_TOOLCHAIN == no

rtld's Makefile used to add -L${LIBDIR} to LDFLAGS when MK_TOOLCHAIN was
no. This was done as part of a change to fix building rtld with
MK_TOOLCHAIN == no (although I'm not sure this part was necessary).

In any case as of 5f2e84015da7 libc_pic.a is built independent of the
MK_TOOLCHAIN setting and the main part of the workaround has already
been removed. Remove the rest now.

This reverts commit c0f5aeb0329d71e6b02379133c0c9c0145c9afea.

Reviewed by: jrtc27
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39938