remove duplicate section select button from react gradebook
remove duplicate section select button from react gradebook

In React Gradebook, there should only be one section select
button. Previously, there were 2, one in the header-buttons
tag, and one in the gradebook-toolbar tag. Now there is
only 1, in the gradebook-toolbar tag.

closes CNVS-27718

test plan:
1. Create a published course with more than 1 section in
   current gradebook.
2. Create a bunch of assignments and enroll students in
   each of the sections you created.
3. Load current gradebook.
4. Notice there is only one section drop-down in upper right.
5. Turn on Gradebook Performance feature flag.
6. Load gradebook again.
7. There should now only be one section drop down next to
   the search bar on the left.

Change-Id: I343d2c84b6b3d448b306c34a5713bc0907ca028c
Reviewed-on: https://gerrit.instructure.com/77520
Tested-by: Jenkins
Reviewed-by: Shahbaz Javeed <6692d7be3fa215e5483a121d6fb314def6049877@instructure.com>
QA-Review: Amber Taniuchi <ef4bfd1e6d278f6bc7d4a478f02be2cb9581a0a5@instructure.com>
Product-Review: Christi Wruck

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