Enable --use-header by default and remove the corresponding command-line option.

Just as stated.

The current behavior of valac is to always generate the header file internally and store it if a header_filename has been set. However, ValaCCodeBaseModule.add_symbol_declaration will ignore this header file if --use-header is not enabled and return false, forcing the various generate_*_definition methods to put symbol definitions in the C files.

Here I update ValaCodeContext.use_header to evaluate as true whenever a header file exists, so that add_symbol_declaration and other files that check for a header will behave appropriately.