[removal] Remove too strict PreferInlineTraitRule and its collectors

[TomasVotruba]

Why

PreferInlineTraitRule nagged to inline any trait used at most maxUsage times (2 by default). Too strict - a small shared trait is a legitimate way to compose behavior, and "inline it" is a style opinion, not a bug.

It also sits oddly next to the package's own stance of discouraging traits in the first place.

What

• Remove PreferInlineTraitRule, its symplify.preferInlineTrait identifier, and its test.
• Remove the two collectors it was the sole user of: TraitDefinitionCollector, TraitUsageCollector.
• Remove config/inline-trait-rules.neon and its composer.json include entry.
• Remove the inlineTrait toggle parameter from config/phpstan-extensions.neon.
• Remove its README section.

Checks

ECS, PHPStan, Rector, Neon lint and PHPUnit all green.