aconfig: call Vec::retain instead of Vec::retain_mut

Replace a call to Vec::retain_mut with a call to Vec::retain instead:
the predicate used doesn't require its parameter to be a *mutable*
reference.

Bug: N/A
Test: atest aconfig.test
Change-Id: Iacd5f2ebef358ebc92ae6a1a78dd3045e44196f4