tombstoned: allow unlinking anr files

Tombstoned unlinks "trace_XX" files if there are too many of them.

avc: denied { unlink } for comm="tombstoned" name="trace_12"
scontext=u:r:tombstoned:s0 tcontext=u:object_r:anr_data_file:s0
tclass=file

Bug: 77970585
Test: Build/boot taimen. adb root; sigquit an app.
Change-Id: I2c7cf81a837d82c4960c4c666b38cd910885d78d

public/tombstoned.te

@@ -19,4 +19,4 @@ auditallow tombstoned anr_data_file:file { append write };
 # Changes for the new stack dumping mechanism. Each trace goes into a
 # separate file, and these files are managed by tombstoned.
 allow tombstoned anr_data_file:dir rw_dir_perms;
-allow tombstoned anr_data_file:file { getattr open create };
+allow tombstoned anr_data_file:file { create getattr open unlink };