generic_x86: delete system_server execmem

https://android-review.googlesource.com/175922 removed all uses
of system_server execmem and neverallowed it. The x86 emulator policy
inappropriately includes this rule. Delete it.

Fixes the following build breakage:

  libsepol.report_failure: neverallow on line 473 of external/sepolicy/system_server.te (or line 12452 of policy.conf) violated by allow system_server system_server:process { execmem };
  libsepol.check_assertions: 1 neverallow failures occurred
  Error while expanding policy

Change-Id: I7fbfaa0a09e8f4e8a372d2f1a64bbe58d5302204