system_linker_exec: Remove exec_type

system/sepolicy/public/attributes defines exec_type as:

  # All types used for domain entry points.
  attribute exec_type;

The linker is not a standalone executable, but rather, used by other
executables to resolve shared libraries. It isn't, and must never be, an
allowed entrypoint for a domain.

Remove the exec_type attribute from system_linker_exec.

Test: Device compiles and boots, no obvious problems running the system.
Change-Id: I8f2f608bc1a642193524396f46b22933faac5468

public/file.te

@@ -135,7 +135,7 @@ type system_file, file_type;
 # Default type for anything under /system/lib[64].
 type system_lib_file, file_type;
 # Default type for linker executable /system/bin/linker[64].
-type system_linker_exec, exec_type, file_type;
+type system_linker_exec, file_type;
 # Default type for linker config /system/etc/ld.config.*.
 type system_linker_config_file, file_type;
 # Default type for linker config /system/etc/seccomp_policy/*.