Remove unused code system/stack/l2cap/l2c_link

Bug: 191555414
Tag: #refactor
Test: gd/cert/run
BYPASS_LONG_LINES_REASON: Bluetooth likes 120 lines
Ignore-AOSP-First: Cherrypick

Original change:
https://android-review.googlesource.com/c/platform/packages/modules/Bluetooth/+/2084123

Change-Id: Ia2ae5322246c0553aafc084e0cc9f900f0e5ca5c
Merged-In: Ia2ae5322246c0553aafc084e0cc9f900f0e5ca5c