scheduler: revert low space threshold hard limit (#2864)

[ti-srebot]

cherry-pick #2864 to release-4.0

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What problem does this PR solve?

The previous PR uses a hard limit for low space. If the capacity beyond 500G, it will use 100G as its low space threshold. It doesn't seem reasonable.

What is changed and how it works?

This PR changes back to the original way to control low space.

Check List

Tests

• Unit test

Related changes

• Need to cherry-pick to the release branch

Release note

• Change back to the original way to control the low space threshold

[disksing]

/merge

[ti-srebot]

/run-all-tests

[codecov]

Codecov Report

Merging #2875 into release-4.0 will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release-4.0    #2875      +/-   ##
===============================================
- Coverage        77.37%   77.37%   -0.01%     
===============================================
  Files              208      208              
  Lines            22765    22762       -3     
===============================================
- Hits             17615    17612       -3     
+ Misses            3822     3816       -6     
- Partials          1328     1334       +6     

Impacted Files	Coverage Δ
server/core/store.go	80.12% <100.00%> (-0.19%)	⬇️
server/member/leader.go	71.20% <0.00%> (-1.95%)	⬇️
server/tso/tso.go	78.48% <0.00%> (-1.90%)	⬇️
server/region_syncer/client.go	85.40% <0.00%> (-1.46%)	⬇️
server/server.go	76.37% <0.00%> (-0.75%)	⬇️
client/base_client.go	90.60% <0.00%> (-0.68%)	⬇️
server/schedule/operator_controller.go	81.41% <0.00%> (+0.16%)	⬆️
pkg/btree/btree.go	87.65% <0.00%> (+0.80%)	⬆️
server/schedulers/adjacent_region.go	76.59% <0.00%> (+1.06%)	⬆️
pkg/mock/mockhbstream/mockhbstream.go	90.76% <0.00%> (+1.53%)	⬆️
... and 3 more

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[ti-srebot]

@ti-srebot merge failed.

[zz-jason]

LGTM