init: remove _SYS_INIT_LEVEL* definitions

The _SYS_INIT_LEVEL* definitions were used to indicate the index entry
into the levels array defined in init.c (z_sys_init_run_level). init.c
uses this information internally, so there is no point in exposing this
in a public header. It has been replaced with an enum inside init.c. The
device shell was re-using the same defines to index its own array. This
is a fragile design, the shell needs to be responsible of its own data
indexing. A similar situation happened with some unit tests.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>