Handler class does not have a `watch` field

[Luthaf]

Description

Remove extraneous field assignment. This breaks the build for me when trying to compile the master branch using https://github.com/plumed/conda/.

Since this was not caught in the 6 months since this code was introduced, maybe it is not used anywhere in the code base? I'm not sure why this is failing now.

Type of contribution

• [x] changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
• [ ] changes to a module not authored by you
• [ ] new module contribution or edit of a module authored by you

Copyright

• [x] I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

• [ ] the module I added or modified contains a COPYRIGHT file with the correct license information. Code should be released under an open source license. I also used the command cd src && ./header.sh mymodulename in order to make sure the headers of the module are correct.

Tests

• [ ] I added a new regtest or modified an existing regtest to validate my changes.
• [ ] I verified that all regtests are passed successfully on GitHub Actions.

[GiovanniBussi]

Yes I think it's not used. Actually I think it should be:

handler.section=nullptr;

Can you confirm that this would compile correctly? Thanks!

[Luthaf]

Yes, this builds correctly with section!