Remove system_current from update_prebuilts.py

I894194fbeae58d73b51c2475d0c0b507bcfdfb6d removed the system_current
directory, which breaks any update_prebuilts.py when it tries to
git add:
fatal: pathspec 'system_current' did not match any files

Test: ./update_prebuilts.py -c 4023242
Change-Id: I9262b152d1749ad417e8cd9cd65ec67c3a46e953
Merged-In: I9262b152d1749ad417e8cd9cd65ec67c3a46e953
(cherry picked from commit 941d980af66375ac56d14ff5b1420a21432ad53d)
1 file changed