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

[emaste]

Summary

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 as far as I can tell this part served no
purpose.  In any case as of 5f2e84015da7 libc_pic.a build is independent
of MK_TOOLCHAIN and the main part of the workaround has already been
removed.  Remove the rest now.

This reverts commit c0f5aeb0329d71e6b02379133c0c9c0145c9afea.

Sponsored by:   The FreeBSD Foundation

[jrtc27]

\o/