fix: Remove unused parameter from QuerySelectMultipleField instantiation

[Dosenpfand]

Description

Remove the unused parameter allow_blank from the instantiation of the QuerySelectMultipleField object to avoid throwing the warning:

fields.py:179: UserWarning: allow_blank=True does not do anything for QuerySelectMultipleField.

ADDITIONAL INFORMATION

• Has associated issue:
• Is CRUD MVC related.
• Is Auth, RBAC security related.
• Changes the security db schema.
• Introduces new feature
• Removes existing feature

[codecov]

Codecov Report

Merging #1991 (97f94e1) into master (3d17741) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1991      +/-   ##
==========================================
- Coverage   78.13%   78.10%   -0.03%     
==========================================
  Files          71       71              
  Lines        8612     8611       -1     
==========================================
- Hits         6729     6726       -3     
- Misses       1883     1885       +2     

Flag	Coverage Δ
python	78.10% <ø> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flask_appbuilder/forms.py	98.16% <ø> (-0.02%)	⬇️
flask_appbuilder/fields.py	69.06% <0.00%> (-1.44%)	⬇️

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[dpgaspar]

Thank you!