remove incorrect setting of result_t

[jmitrevs]

Description

The result_t was incorrect set in the MergeLinearActivation optimizer. This removes the setting, which wasn't needed anyway. Setting get_output_variable().type.precision already sets it properly as a side-effect.

Type of change

• [x] Bug fix (non-breaking change that fixes an issue)

Tests

This fixes a bug found in #1122. The test is there.

Checklist

• [x] I have read the guidelines for contributing.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] My changes generate no new warnings.
• [x] I have installed and run pre-commit on the files I edited or added.
• [ ] I have added tests that prove my fix is effective or that my feature works.

[jmitrevs]

A few more optimizers also needlessly (and incorrectly) set result_t, so I removed those, too.