Remove ADAPTIVE_READ_BUFFERING ifdef

Remove ADAPTIVE_READ_BUFFERING ifdef

* src/process.c (make-process, make-pipe-process, deactivate_process)
(wait_reading_process_output, read_process_output, send_process)
(init_process_emacs): ifdef ADAPTIVE_READ_BUFFERING was originally
added in case there was an operating system in which it was not
useful.  That was 11 years ago and it hasn't happened.  Make
development easier by not considering the effect of changes on a
theoretical OS where this is disabled (Bug#20978).