Fix multiple assignment of "visibility" in generate_library_loader.gni

If the invoker has visibility set, it will be assigned twice, and
rejected by GN. Remove the second assignment.

TBR=thakis

Review URL: https://codereview.chromium.org/1721003003

Cr-Original-Commit-Position: refs/heads/master@{#377090}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6ad2f470bac33901adc68c86e1863cde5985838e