Remove WLAN MMS related deadcode

The deadcode was added in ag/24326258 to allow bring up WLAN mms network even when a
default network that's capable of mms already exists. This code is no
longer used because once QNS preferrs on WLAN, the MMS capability of the
existing network is removed, hence new WLAN MMS network will be brought
up naturally

Test: voice call + data browsing
Bug: cleanup
Change-Id: I7f94e11e968444f1ded8d88d413321291acc04e8