Remove `--strings-unified-vspt` option (#8947)

Remove `--strings-unified-vspt` option (#8947)

This option was enabled by default and this commit enables it
permanently. Overall, the results (300s timeout) seem to indicate that
the option does not have a significant impact on performance (and is a
bit worse overall):

```
                                status  total  solved    sat  unsat   best  timeout  memout  error  uniq  disagr  time_cpu   memory
config
2022-07-07-strings-main-no-vspt     ok  69984   69513  42755  26758  65940      468       0      1    12       0  175814.9 456860.9
2022-07-07-strings-main-vspt        ok  69984   69509  42750  26759  65097      472       0      1     8       0  174975.7 456793.6
```