t/testenv.sh: drop unused configuration variable

The LFS_CONFIG variable, which we set in our t/testenv.sh library,
is not referenced by any part of our current test suite.  The last
use of this variable was removed in 2015 in commit
1feee58 of PR git-lfs#335, when the
test suite was revised to create a private home directory for the
tests, so as to be able to set a standard global Git configuration
separate from that of the current user's configuration.  Hence we
can just remove the LFS_CONFIG variable now.

Author: chrisd8088

t/testenv.sh

@@ -111,10 +111,6 @@ REMOTEDIR="$ROOTDIR/t/remote"
 #
 CREDSDIR="$REMOTEDIR/creds/"
 
-# This is the prefix for Git config files.  See the "Test Suite" section in
-# t/README.md
-LFS_CONFIG="$REMOTEDIR/config"
-
 # This file contains the URL of the test Git server. See the "Test Suite"
 # section in t/README.md
 LFS_URL_FILE="$REMOTEDIR/url"