iflib: Remove redundant variable

In iflib_init_locked(), sctx and scctx both point to the same value,
which is the ifc_softc_ctx field in the iflib softc. Remove the
declaration and assignment to sctx since scctx can be used instead, and
the name of scctx follows the naming convention used for local variables
that point to ifc_softc_ctx.

In theory there should be no functional impact with this change.

Signed-off-by: Eric Joyner <erj@FreeBSD.org>

Reviewed by: kbowling@
MFC after: 3 days
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D41325