Remove unused file from build.
Improve the gdbmi/mi grammar and associated documentation.

The file my_gdbmi_grammar.txt has the explanations of how the
grammar is derived from the corresponding gdb/mi syntax in the
GDB manual. In a few more iterations I'll remove the old
gdbmi_grammar.txt and make this documentation more official.
Improve grammar result_list rule to match gdb/mi syntax more closely.

This makes it easier to understand the documented syntax
in the GDB manual and compare it to our bison parser grammar.
Replace the word kind with choice in grammar documentation.
Treat value-list similar to result-list to increase grammar similarity.
Remove no longer used grammar documentation.
Put new documentation into final destination.