Remove 28.0 compat support

Treble doesn't support U system + P vendor, so removing P (28.0)
prebuilts and compat files.

Bug: 267692547
Test: build
Change-Id: I943a1f70b3600918a375ff62a20d45cfae2f673e
diff --git a/core/config.mk b/core/config.mk
index 91c8bd8..26e90ef 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -863,7 +863,6 @@
 
 # A list of SEPolicy versions, besides PLATFORM_SEPOLICY_VERSION, that the framework supports.
 PLATFORM_SEPOLICY_COMPAT_VERSIONS := \
-    28.0 \
     29.0 \
     30.0 \
     31.0 \