Remove the default last activity date from reports

[m1r0]

This PR removes the default last activity start date. This was causing confusion for some users, so we decided to remove it.

Changes proposed in this Pull Request

• Remove the default last activity start date from all report screens.

Testing instructions

• Go to Sensei LMS -> Reports -> Students.
• Make sure the last activity start date input is empty.
• Go to Sensei LMS -> Reports -> Courses.
• Make sure the last activity start date input is empty.

Screenshot / Video

Context

p1664231630242489-slack-C02P7FHLVR9

[codecov[bot]]

Codecov Report

Merging #5793 (1a27b78) into trunk (0ff1e2c) will decrease coverage by 0.00%. The diff coverage is 47.32%.

:exclamation: Current head 1a27b78 differs from pull request most recent head b3334f4. Consider uploading reports for the commit b3334f4 to get more accurate results

@@             Coverage Diff              @@
##              trunk    #5793      +/-   ##
============================================
- Coverage     44.88%   44.88%   -0.01%     
+ Complexity     8882     8880       -2     
============================================
  Files           430      430              
  Lines         31787    31784       -3     
  Branches        266      266              
============================================
- Hits          14268    14266       -2     
+ Misses        17322    17321       -1     
  Partials        197      197              

Impacted Files	Coverage Δ
...s/admin/editor-wizard/steps/course-upgrade-step.js	72.72% <ø> (ø)
.../admin/editor-wizard/steps/lesson-patterns-step.js	33.33% <ø> (ø)
assets/blocks/quiz/quiz-block/quiz-timer-promo.js	100.00% <ø> (ø)
assets/shared/blocks/settings.js	18.18% <0.00%> (ø)
includes/admin/class-sensei-learner-management.php	2.21% <0.00%> (ø)
includes/admin/class-sensei-learners-main.php	4.96% <0.00%> (ø)
...des/block-patterns/class-sensei-block-patterns.php	27.69% <0.00%> (ø)
...e-list/class-sensei-course-list-block-patterns.php	0.00% <0.00%> (ø)
...cludes/blocks/class-sensei-block-quiz-progress.php	0.00% <0.00%> (ø)
includes/blocks/class-sensei-blocks.php	21.53% <0.00%> (ø)
... and 120 more

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[m1r0]

@Automattic/nexus Could someone review this PR? It should be a quick one. 🙂