kernel/.gitgnore: remove stale timeconst.h and hz.bc

timeconst.h and hz.bc used to exist in kernel/.

Commit 5cee9645 ("time/timers: Move all time(r) related files into
kernel/time") moved them to kernel/time/.

Commit 0a227985

 ("time: Move timeconst.h into include/generated")
moved timeconst.h to include/generated/ and removed hz.bc .

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>