remove 'dead links' in javadocs

[NathanQingyangXu]

not 100% sure about some changes. There are roughly the following reasons of 'dead link's in javadocs:

• previoulsy deprecated classes have been deleted (I provided the latest public javadoc link accessible; usually in v3.0.12)
• outdated reference which should have been deleted when the referenced stuff had been removed (I removed them)
• mystic references which are difficult to trace from even git history (I removed them simply; mainly in testing classes)
• erroneous method signature (I fixed it by using correct signature)

There are a couple of links pointing to private methods in other classes. Not sure how to fix them so I didn't touch them or include them in this PR.

Again, feel free to change directly. Looking forward to corrections.

[artembilan]

Checkstyle violation:

Error: [Task :spring-kafka:checkstyleMain] [Regexp] Line matches the illegal pattern 'Trailing whitespace'.

Consider to run `./gradlew check -x test` locally before pushing to PR.

Thanks