This removes a bunch of unused files that would have been touched by
subsequent commits that switch to using nxt_bool_t (AKA unit6_t) in
structures.

In auto/sources we have

  NXT_LIB_SRC0=" \
      src/nxt_buf_filter.c \
      src/nxt_job_file.c \
      src/nxt_stream_module.c \
      src/nxt_stream_source.c \
      src/nxt_upstream_source.c \
      src/nxt_http_source.c \
      src/nxt_fastcgi_source.c \
      src/nxt_fastcgi_record_parse.c \
  \
      src/nxt_mem_pool_cleanup.h \
      src/nxt_mem_pool_cleanup.c \
  "

None of these seem to actually be used anywhere (other than within
themselves). That variable is _not_ referenced anywhere else.

Also remove the unused related header files: src/nxt_buf_filter.h,
src/nxt_fastcgi_source.h, src/nxt_http_source.h, src/nxt_job_file.h,
src/nxt_stream_source.h and src/nxt_upstream_source.h

Also, these files do not seem to be used, no mention under auto/ or build/

  src/nxt_file_cache.c
  src/nxt_cache.c
  src/nxt_job_file_cache.c

src/nxt_cache.h is #included in src/nxt_main.h, but AFAICT is not
actually used.

With all the above removed

  $ ./configure --openssl --debug --tests && make -j && make -j tests &&
  make libnxt

all builds.

Buildbot passes.

NOTE: You may need to do a 'make clean' before the next build attempt.

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>