lib/helpers: remove log message from _command_exists() et al

[gaelicWizard]

Description

Completely remove all logging from _command_exists(), _binary_exists(), _completion_exists(), and _is_function().

• and move them to lib/utilities.

Motivation and Context

These functions are used everywhere and both junk up bash-it doctor, but can alsö add unexpected strings to some theme prompt lines!

It does not seem like the logging was ever used anywhere prior to my converting the codebase to use it, although it was pointed out that this is often used to short-circuit plugin loading and so the default message could be helpful in bash-it doctor. I have a separate PR which adds actual logging to plugins that short-circuit.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[seefood]

looks ok to me, unless I am missing something big. Isn't this ready for merging?