hrw4u: grammar accepts bare operator statements (`no-op;`) that can never compile

[masaori335]

Summary

The grammar has a statement alternative for a bare operator without parentheses:

trafficserver/tools/hrw4u/grammar/hrw4u.g4

Line 171 in 13ef6a7

| op=IDENT SEMICOLON

Nothing written that way compiles. Every op is rejected, so the alternative is
unreachable and should be removed from the grammar.

Reproduce

source	result
no-op();	no-op
no-op;	error: This operator requires an argument
skip-remap;, set-debug;	same error

Why remove rather than fix

The call form is the only form anything produces or documents:

producer	form
doc/admin-guide/configuration/hrw4u.en.rst	no-op();, set-debug(), skip-remap()
reverse conversion, u4wrh (src/hrw_symbols.py:315)	always name()
test corpus (tests/data)	no-op();, 0 bare-op inputs

Making the bare form work instead (check arity rather than validator presence)
would add a second spelling for the same statement with no caller asking for
it. break; is a separate alternative and is unaffected either way.

Trade-off: after removal, no-op; becomes an ANTLR syntax error rather than a
named one. If the friendlier message matters, it belongs in the suggestion
engine, not in a grammar alternative that produces no output.

Note

test_the_corpus_reaches_every_grammar_rule does not catch this: it records
rule names, and statement is reached through its other alternatives.