Removing the should from Exception.h

Description

I encoundered a case i which the if (!(test)) plumed_error() << PLMD::Exception::Assertion(#test), the plumed_dbg_massert macro expansion with NDEBUG, is not optimized away, and this patch is the least invasive thing to do:

if constexpr(false) is guaranteed to be evaluated at compile time, so that the rest of the line is ignored.

I think helps in improving the code optimization after inlining a function, at least that is what I think it happened in my case

Target release

I would like my code to appear in release 2.10/2.9 (this is a single commit, it can be easily rebased on the 2.9)

Type of contribution

[ ] 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.

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (9c3246e) 84.08% compared to head (72b139d) 84.08%. Report is 9 commits behind head on master.

:exclamation: Current head 72b139d differs from pull request most recent head 03074ed. Consider uploading reports for the commit 03074ed to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1007 +/- ## ======================================= Coverage 84.08% 84.08% ======================================= Files 602 602 Lines 56246 56246 ======================================= Hits 47297 47297 Misses 8949 8949 ```

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plumed / plumed2