[removal] Remove too strict and unregistered NoProtectedClassStmtRule

[TomasVotruba]

Why

NoProtectedClassStmtRule forbade every protected member ("use clear interface contract instead"). Two problems:

• Too strict - protected is a normal, legitimate visibility; banning it wholesale is dogmatic.
• Unreachable - the rule was registered in no shipped config and not listed in composer.json extra.phpstan.includes, so no user ever ran it. It shipped as dead code plus a test.

What

• Remove the rule, its test, and its symplify.noProtectedClassStmt identifier.
• Drop the now-unused MethodName::SET_UP and MethodName::TEAR_DOWN constants (only this rule used them).
• Remove its README section.

Checks

ECS, PHPStan, Rector and PHPUnit all green.