Cleanup and convert miscellaneous checks to eassert.

Cleanup and convert miscellaneous checks to eassert.
* alloc.c (mark_interval): Fix comment, partially rephrase
old comment from intervals.h (see below).
* intervals.c (find_interval, adjust_intervals_for_insertion)
(delete_interval, adjust_intervals_for_deletion)
(graft_intervals_into_buffer, temp_set_point_both, copy_intervals):
Convert to eassert.
(adjust_intervals_for_insertion, make_new_interval):
Remove obsolete and unused code.
* intervals.h (struct interval): Remove obsolete comment.
* textprotp.c (erase_properties): Remove unused code.
(Fadd_text_properties, set_text_properties_1, Fremove_text_properties)
(Fremove_list_of_text_properties): Convert to eassert.