[cling] remove remnant declaration of removed function

[ferdymercury]

This Pull request:

Changes or fixes:

Function was removed here https://github.com/root-project/cling/commit/c23abbc88b6ee61b966291c785794743b1bd5c62

Fixes https://github.com/root-project/cling/issues/456

Checklist:

• [ ] tested changes locally
• [ ] updated the docs (if necessary)

[github-actions[bot]]

Test Results

    12 files      12 suites   2d 5h 35m 50s :stopwatch:  2 609 tests  2 609 :white_check_mark: 0 :zzz: 0 :x: 29 374 runs  29 374 :white_check_mark: 0 :zzz: 0 :x:

Results for commit b32807a6.

:recycle: This comment has been updated with latest results.