Remove hardcoded <kernel> version from fwk comp mat

They are injected via assemble_vintf by the build system
at build time. See build/make/target/board/Android.mk.

Test: m system_compatibility_matrix.xml, and manually
inspect $OUT/system/compatibility_matrix.xml; it contains
<kernel> versions.

Bug: 64124223

Change-Id: Ibf82efa0805233dedb6badce7e5a4ff62a9ff769
Merged-In: Ibf82efa0805233dedb6badce7e5a4ff62a9ff769