Remove unused routine assert_lacc_equals_i_am_accel_node

What is the new feature

It's not super urgent to remove this routine, but assert_lacc_equals_i_am_accel_node is not used in ICON any more.

How is it implemented

grep -r assert_lacc_equals_i_am_accel_node in icon only found the definition and test in libfortran-support.

Mandatory steps before review

• Gitlab CI passes (Hint: use make format for linting)
• New feature is covered by additional unit tests
• Mark the merge request as ready by removing Draft:

Mandatory steps before merge

• Test coverage does not decrease
• Reviewed by a maintainer
• Incorporate review suggestions
• Prior to merging, please remove any boilerplate from the MR description, retaining only the Please describe your feature in a couple of words and describe important implementation details of the feature section to maintain
• Remember to edit the commit message and select the proper changelog category (feature/bugfix/other)

You are not supposed to merge this request by yourself, the maintainers of fortan-support take care of this action!