Remove codec libs from base_system.mk

- libstagefright_enc_common
- libstagefright_amrnb_common

Bug: 141885505
Test: build and boot.
      Checked libstagefright_enc_common.so doesn't exist in /system/lib64.
      libstagefright_amrnb_common.so is still there because other system
      module, librtp_jni.so, is using it, but there is no reason to keep
      this in base_system.mk.

Change-Id: I43692b50bd23e0b857606b42cb000c8566489dd6