kernel: drop 4.18 bbappend
kernel: drop 4.18 bbappend

4.18 has been removed from oe-core master, so we no longer need
this bbappend.

Signed-off-by: Bruce Ashfield <284cc737d174a260eec2a2a6324a304f84b1ae0f@gmail.com>

lgirdk/meta-virtualization,lgirdk/meta-virtualization,lgirdk/meta-virtualization,lgirdk/meta-virtualization,lgirdk/meta-virtualization