share: fix misleading 0 cells log message

[widlarizer]

Prior to this PR, yosys would always print "0 cells" in the affected log message since sat_cells is an otherwise unused object. This may mislead users about the SAT being stuck thinking about nothing. This PR removes this alleged cell count instead. There is no replacement cell count implemented, open to suggestions or followups.

../../yosys share.ys | grep "Size of SAT" demonstrates this well:

      Size of SAT problem: 0 cells, 8 variables, 19 clauses
      Size of SAT problem: 0 cells, 8 variables, 19 clauses
      Size of SAT problem: 0 cells, 240 variables, 617 clauses
      Size of SAT problem: 0 cells, 236 variables, 605 clauses
      Size of SAT problem: 0 cells, 245 variables, 632 clauses

Credit for digging up the report to @JulianKemmerer