mediaex: remove invalid syscall from arm64 seccomp

Addresses:
09-20 19:41:17.327   694   694 W /system/bin/mediaextractor:
libminijail[694]: compile_file: nonexistent syscall '_llseek'

Bug: 66084343
Test: boot marlin without the above warning.
Change-Id: I7daafa448b63d9f939ad59e3c9de6c4219697c56