Remove unused stack_alloc.h

I was going through trying to figure out where the different configuration checks were being used, and I noticed that libspeexdsp/stack_alloc.h isn't actually used anywhere in this project. I assume it came from libspeex and may have been used at some point, but it isn't used here from what I can see. The header appears to relate to handling stack allocations using variable-length arrays or alloca depending on compiler support determined by the configure script. However, the header is never used, and the few locations that can be optimized by it just uses conditionally-compiled variable-length arrays.

This is actually one of three header files I found that don't appear to be used, with the others being _kiss_fft_guts.h and misc_bfin.h. However, I haven't look as much into what purpose those files have (or had), and I decided to make a MR for removing this one file first.

Removing this header file also allows removing code from the autoconf configuration related to it. stack_alloc.h was the only file that used the macros USE_ALLOCA, HAVE_ALLOCA_H, and ENABLE_VALGRIND. By removing this header, these can also be removed from the configuration, which helps simplify the build process. Also, I found a check for getopt.h that isn't being used in the project (likely copied from libspeex and its command-line tools), so I removed it as well.