fix: remove unused fixedWithdrawAmountSat import

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ben-kaufman

app/src/main/java/to/bitkit/viewmodels/AppViewModel.kt

@@ -78,7 +78,6 @@ import to.bitkit.ext.claimableAtHeight
 import to.bitkit.ext.getClipboardText
 import to.bitkit.ext.getSatsPerVByteFor
 import to.bitkit.ext.callbackAmountMsats
-import to.bitkit.ext.fixedWithdrawAmountSat
 import to.bitkit.ext.isFixedAmount
 import to.bitkit.ext.maxSendableSat
 import to.bitkit.ext.maxWithdrawableSat