Remove //apex_available:platform from libc's apex_available

`libc` is part of runtime apex now

Bug: 147364041
Test: m libc-checkbuild com.android.runtime

Change-Id: I24e502c15085b5d7d607fd26de1f83eb4a0e66ff

Author: dasspandan

libc/Android.bp

@@ -1656,7 +1656,6 @@ cc_defaults {
     },
 
     apex_available: [
-        "//apex_available:platform",
         "com.android.runtime",
     ],