Remove LOW_MEMORY_NODE and CLEAR_VOTES

• Added a fatal error in CMake informing about the deprecation of LOW_MEMORY_NODE option, which is displayed after passing the option LOW_MEMORY_NODE=OFF (also added a warning when the option is set to ON) (same applied to the CLEAR_VOTES option)
• Made STORE_ACCOUNT_METADATA option independent from LOW_MEMORY_NODE in CMake. It is now set by default to ON.
• Added a note in docs about the deprecation of LOW_MEMORY_NODE and CLEAR_VOTES
• Added a reminder in smt_token_object that indexing by contributor might cause optimization problems in the future
• Removed LOW_MEMORY_NODE and CLEAR_VOTES #ifdefs and #ifndefs from the code
• Removed LOW_MEMORY_NODE and CLEAR_VOTES options from the tests/README.md and CI scripts
• Fixed comparison of different signedness in the delayed voting tests
• Added -r switch to the xargs kill command in the run_proposal_tests.py file to prevent displaying unnecessary help messages