shill: cellular: Remove redundant log messages.

BeginAsyncDBusCall already logs a trace message via SLOG(DBus, 2). This
CL removes those redundant SLOG(Modem, 2) messages that appear before
BeginAsyncDBusCall.

BUG=None
TEST=`USE=cellular FEATURES=test emerge-$BOARD platform2`

Change-Id: I5a472ffbc9786102942866a9dcb850922bdad3a4
Reviewed-on: https://chromium-review.googlesource.com/196467
Reviewed-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>