fix!: restrict MsgWithdraw to only accept uToken inputs

[toteki]

Description

Removes the ability of MsgWithdraw to convert base token inputs to uTokens automatically.

This fixes a very minor problem, where the double conversion of input token -> uToken -> token would result in a final token amount which had been rounded down twice as opposed to once when users input a token amount, e.g. 1000uumee -> 999u/uumee -> 998uumee.

With this fix, user input is denominated in uTokens, so the rounding is minimized to the necessary amount.

Runtime verification had mentioned this in a "functional correctness" issue at one point.

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[toteki]

@AgentRoberto This will impact frontend, so share any thoughts

[codecov-commenter]

Codecov Report

Merging #1023 (5207426) into main (30bca26) will decrease coverage by 0.02%.
The diff coverage is 68.75%.

@@            Coverage Diff             @@
##             main    #1023      +/-   ##
==========================================
- Coverage   43.86%   43.83%   -0.03%     
==========================================
  Files          64       64              
  Lines        8337     8322      -15     
==========================================
- Hits         3657     3648       -9     
+ Misses       4424     4419       -5     
+ Partials      256      255       -1     

Impacted Files	Coverage Δ
x/leverage/keeper/keeper.go	45.33% <66.66%> (-0.55%)	⬇️
x/leverage/client/tests/tests.go	100.00% <100.00%> (ø)

[robert-zaremba]

let's rename amount.

[robert-zaremba]

seams like we had a potential bug here if we the function doesn't handle case where uToken == withdrawal.