Remove parser_state "initial_size" parameter

Remove parser_state "initial_size" parameter

All the real (not test) uses of parser_state pass 10 as the
"initial_size" parameter, and it seems to me that there's no real
reason to require callers to set this.  This patch removes this
parameter.

gdb/ChangeLog
2019-04-04  Tom Tromey  <tom@tromey.com>

	* dtrace-probe.c (dtrace_probe::build_arg_exprs): Update.
	* stap-probe.c (stap_parse_argument): Update.
	* stap-probe.h (struct stap_parse_info) <stap_parse_info>: Remove
	initial_size parameter.
	* rust-exp.y (rust_lex_tests): Update.
	* parse.c (parser_state): Update.
	(parse_exp_in_context): Update.
	* parser-defs.h (struct parser_state) <parser_state>: Remove
	"initial_size" parameter.