(core) Remove messytables completely, particularly for excel imports

Summary: Mirror of https://github.com/gristlabs/grist-core/pull/289

Test Plan: In addition to the PR, tweaked one Excel fixture file and tests involving that and one other fixture.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3640

sandbox/grist/imports/import_utils.py

@@ -24,7 +24,7 @@ def column_count_modal(rows):
     length = len([c for c in row if not empty(c)])
     if length > 1:
       counts[length] += 1
-  if not len(counts):
+  if not counts:
     return 0
   return max(list(counts.items()), key=lambda k_v: k_v[1])[0]