shill: cellular: Remove a runaway update to Cellular::home_provider.

All old code paths that directly updated |Cellular::home_provider| from the
capabilities were removed by crosreview.com/198322, but we missed one.
This CL removes it.

BUG=chromium:375573
TEST=On a CDMA Gobi3K modem, ensure that home provider is determined correctly.

Change-Id: I2dff86a287008a09f15ebea7cbc13bf21f31d3fb
Reviewed-on: https://chromium-review.googlesource.com/200676
Reviewed-by: Thieu Le <thieule@chromium.org>
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>