Fix Wunused-variable warnings

Issue/feature addressed

Some variables are left unused in some files in the solvers/ directory. For some reasons those warnings are not detected on the CI. Those warnings appear by using the -Wall -Wextra flags when compiling on intel compiler 2022.

Proposed solution

Delete unused variables. Fixing this issue will eventually allow a higher level of warning to be used at compilation without being flooded by warning messages.

Checklist

• [ ] Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• [ ] Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).

Warning

On the 19.07 the code formatting (code style) was standardised using clang-format, over the whole Nektar++ code. This means changes in your branch will conflict with formatting changes on the master branch. To resolve these conflicts , see #295 (closed)