Remove deprecated legacy like kernels in arrow-string (#9674)

# Which issue does this PR close?

- Closes #9675 

# Rationale for this change

The legacy kernels have been deprecated for 2 years and hidden from
docs, and its just more code to build.

# What changes are included in this PR?

1. Removing long deprecated legacy like kernels
2. Keeps all tests, just folded them into the existing pattern. This
also increases the effective coverage - testing scalar comparison both
for Utf8 scalars and Dict scalars

# Are these changes tested?

No functional changes, but refactors existing tests

# Are there any user-facing changes?

Only for users using long hidden and deprecated functions, but I this
isn't a meaningfully public API. The functionality is also still
available with very minor changes.

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: AdamGS