Not adding -g flag when compiled with NDEBUG

[XiaowenHu96]

We have NDEBUG = off by default and it allows only a few asserts in the program.
Since #2098, -g is added automatically if NDEBUG is off.
If the user compiled Souffle with the default option, using -c flag willl always results in the target binary being compiled with -g -O0.
I think this behaviour is probably counter-intuitive (unexpected performance regression) and probably not what NDEBUG was intended for.

This PR removes this behaviour. Users that want the debug option should compile the datalog program using souffle-compile.py and pass the flag explicitly.

[codecov]

Codecov Report

Merging #2238 (d98a78a) into master (10a8601) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2238      +/-   ##
==========================================
+ Coverage   75.81%   75.83%   +0.01%     
==========================================
  Files         454      454              
  Lines       27587    27588       +1     
==========================================
+ Hits        20914    20920       +6     
+ Misses       6673     6668       -5     

Impacted Files	Coverage Δ
src/include/souffle/profile/Reader.h	0.00% <0.00%> (ø)
src/main.cpp	67.43% <ø> (-0.10%)	⬇️
src/include/souffle/utility/ParallelUtil.h	85.27% <0.00%> (+1.55%)	⬆️
src/include/souffle/datastructure/UnionFind.h	97.80% <0.00%> (+2.19%)	⬆️
...ouffle/datastructure/ConcurrentInsertOnlyHashMap.h	83.73% <0.00%> (+2.43%)	⬆️