[Variant] Remove unused output builder files (#8320)

# Which issue does this PR close?

* Quick-follow to #8280

# Rationale for this change

The change from output builders to row builders left some no-longer-used
files behind.

# What changes are included in this PR?

Delete the unused files.

# Are these changes tested?

N/A

# Are there any user-facing changes?

No

Author: scovich