arch: arm: Inline arch_isr_direct_header.

This commit inlines arch_isr_direct_header function that was previously
placed in irq_manage.c for no good reason (possibly in relation to the
FIXME for #3056).

In addition, since the PR #20119 resolved the header circular
dependency issue described in the issue #3056, this commit removes the
references to it in the code.

The reason for not inlining _arch_is_direct_pm as the #3056 FIXME
suggests is that there is little to gain from doing so and there still
exists circular dependency for the headers required by this function
(#20119 only addresses kernel_structs.h, which is required for _current
and _kernel, which, in turn, is required for handling interrupt nesting
in many architectures; in fact, Cortex-A and Cortex-R port will require
it as well).

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>