fix(MariaDBTable): dont attempt to drop index twice

[sagarvora]

Prevent error when syncing columns that have both a unique index AND a non-unique one:

In [1]: frappe.db.sql("alter table `tabInstalled Application` add index app_name
   ...: _index(app_name)")
Out[1]: ()

In [2]: frappe.db.sql("alter table `tabInstalled Application` add unique index a
   ...: pp_name_index_(app_name)")
Out[2]: ()

In [3]: from frappe.database.mariadb.schema import MariaDBTable

In [4]: x = MariaDBTable("Installed Application")

In [5]: x.validate()

In [6]: x.sync()
Failed to alter schema using query: ALTER TABLE `tabInstalled Application` DROP INDEX `app_name_index_`, DROP INDEX `app_name_index`, DROP INDEX `app_name_index_`, DROP INDEX `app_name_index`

--- snip traceback ---

Issue encountered when upgrading a site to version 14.

[codecov]

Codecov Report

Merging #19783 (e8e1ed0) into develop (fb9ae08) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #19783      +/-   ##
===========================================
- Coverage    62.75%   62.75%   -0.01%     
===========================================
  Files          754      754              
  Lines        71764    71764              
  Branches      6134     6134              
===========================================
- Hits         45036    45035       -1     
- Misses       23252    23253       +1     
  Partials      3476     3476              

Flag	Coverage Δ
server	68.70% <100.00%> (-0.02%)	⬇️

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[ankush]

that have both a unique index AND a non-unique one

We prevent this from happening right? 🥲

frappe/frappe/core/doctype/doctype/doctype.py

Lines 381 to 383 in fb9ae08

	# unique is automatically an index
	if d.unique:
	d.search_index = 0

[sagarvora]

We prevent this from happening right?

This is the actual query encountered during migration:

ALTER TABLE `tabBank Account` DROP INDEX `account_name_2`, DROP INDEX `account_name`, DROP INDEX `account_name_2`, DROP INDEX `account_name`

Not sure if it is a user-specific issue.

[ankush]

@sagarvora looks like it's this: #19291

Anyway this PR makes sense. Another alternative: run one alter query at time.