Removed a should from Exception.h

[Iximiel]

Description

At first look this may look like an aestetic change.
But 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 I think 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

• 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.