You can remove it, but it requires a SQL editor.
Go to data/data/com.android.providers.telephony/databases and edit nwk_info.db
Select the "carriers" table and locate the T-Mobile US LTE / 310260 entry (should be line 26)
Scroll all the way to the far right and change the editable value from 0 to 1.
Restart your phone and you should be able to edit the entry.
The other option is to edit /system/etc/apns-conf.xml directly.
Remember the first rule in tweaking your system... make a backup copy first.
Samsung Galaxy Note 3 (T-Mobile - Stock 4.4.2-Rooted)
Samsung GS4 (AT&T - Stock 4.4.2-FMLD)
Samsung GS3 (AT&T - Stock 4.3-Rooted)
Samsung GS2 Skyrocket (CM 10.1)
Nokia Lumia 520 (Cool phone to play with)