chore: remove transient store that is not used now

[egonspace]

Description

TransientStore was used only in param store.
However, in the PR #202, the ParamStore no longer uses the TransientStore, making it unnecessary. A lot of code for this could be removed, so I think it's better to erase it.

closes: #XXXX

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #203 (368117e) into v2/develop (88ab1bd) will decrease coverage by 0.01%.
The diff coverage is 75.00%.

@@              Coverage Diff               @@
##           v2/develop     #203      +/-   ##
==============================================
- Coverage       53.49%   53.47%   -0.02%     
==============================================
  Files             653      652       -1     
  Lines           47376    47339      -37     
==============================================
- Hits            25343    25315      -28     
+ Misses          19178    19170       -8     
+ Partials         2855     2854       -1     

Impacted Files	Coverage Δ
baseapp/baseapp.go	77.94% <ø> (+0.85%)	⬆️
store/rootmulti/store.go	65.25% <0.00%> (-0.72%)	⬇️
store/types/gas.go	75.00% <ø> (+1.82%)	⬆️
store/types/store.go	58.82% <ø> (-1.18%)	⬇️
types/context.go	96.15% <ø> (+1.21%)	⬆️
types/store.go	56.25% <ø> (-8.75%)	⬇️
x/params/types/subspace.go	74.78% <ø> (-0.22%)	⬇️
simapp/app.go	83.96% <100.00%> (+0.16%)	⬆️
x/params/keeper/keeper.go	72.22% <100.00%> (-1.47%)	⬇️

[whylee259]

LGTM