Make dropTableColumns drop columns on sqlite and constraints on all

[zeripath]

In #6823 it was suggested that I use migrations.dropTableColumns to fix v78.go. Unfortunately, this is not completely usable as it doesn't drop constraints by default (except on MSSQL).

Further on SQLite it doesn't drop columns.

This PR adjusts the code to allow recreation of the SQLite table to drop columns and adds removal of index constraints for all databases.

I believe that this should work for all variants but I would appreciate reviews.

[codecov-io]

Codecov Report

Merging #6849 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #6849      +/-   ##
==========================================
- Coverage    41.3%   41.26%   -0.04%     
==========================================
  Files         466      466              
  Lines       63186    63254      +68     
==========================================
+ Hits        26099    26102       +3     
- Misses      33676    33742      +66     
+ Partials     3411     3410       -1

Impacted Files	Coverage Δ
models/migrations/migrations.go	1.32% <0%> (-0.2%)	⬇️
modules/process/manager.go	81.15% <0%> (+4.34%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5f25558...1774e78. Read the comment docs.

[zeripath]

This is a necessary step to help fix #3265

[lunny]

@zeripath maybe you can send this PR to xorm and add some unit tests there.

[lunny]

This PR really needs a unit test.