Fix separator inside print

[xaerru]

Fix #2677 and #2668.

I think global_sep_space isn't needed because we set the sep to " " as the default when it's not specified.

I couldn't figure out why that if condition of two consecutive non characters was in place and also why sep_no_space was used.

When the if condition of two consecutive non characters was false, the empty separator was being pushed.

[kmr-srbh]

Thanks for the changes @xaerru! LPython is a sister project of a modern Fortran compiler LFortran. In Fortran, we do not add a space between 2 strings implicitly. We add a space only when either the left value or right value is something other than a string. The deleted code handled the separator based on this criteria.

Please see the discussion at https://github.com/lcompilers/lpython/pull/2670/files#r1581397608 on suggestion to fix this.

[xaerru]

In LFortran ASR, Print node was refactored to only contain StringFormat or a single StringConstant.

Also for StringFormat, only FormatFortran is implemented as of now.

lfortran/lfortran#2543

lfortran/lfortran#4696

I think this should be done after libasr in LPython is synced to LFortran.