Clean up the unused `build_in_import_modules` dictionary in ni-measurement-plugin-client-generator

[jayaseelan-james]

Tech Debt

As discussed in this comment.

In #871, we hard-coded the 'import pathlib', but continued iterating through the built_in_import_modules dictionary, which only contained the 'import pathlib' string literal until it was removed in #900. The use of built_in_import_modules was eliminated in #931. As a result, the built_in_import_modules dictionary and the _PATH_IMPORT variable are no longer used or required and should be removed.

AB#2903265