emerge-delta-webrsync: fix bug #506192

This fixes the sync_local function so that it doesn't prematurely remove
the whole TMPDIR when tarsync is not installed. In this case, we really
only want to remove the "${TMPDIR}"/portage directory which contains a
temporary copy of the portage tree. The faulty code was introduced in
commit 24f6a9599bcf445c468022264cd6952aad4d4076 when a bunch of code
from emerge-webrsync was merged into emerge-delta-webrsync.

Fixes: 24f6a9599bcf ("emerge-delta-webrsync: check full snapshot sig")
X-Gentoo-Bug: 506192
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=506192