chore: Remove unused imports (Task 4)

Task 4: Additional dead code and unused imports cleanup
- Removed 7 unused imports across 6 files:
  - conversion_pipeline.rs: MirModule
  - pattern1_minimal.rs: BTreeMap
  - pattern2_with_break.rs: BTreeMap
  - pattern3_with_if_phi.rs: BTreeMap
  - pattern4_with_continue.rs: CarrierInfo, BTreeMap
  - condition_var_analyzer.rs: BasicBlockId
- Build verified: 0 unused import warnings remaining
- Warning count reduced: 70 → 63 warnings

Result: Cleaner code, reduced compiler warnings

src/mir/loop_pattern_detection/condition_var_analyzer.rs

@@ -15,7 +15,6 @@
 
 use crate::ast::ASTNode;
 use crate::mir::join_ir::lowering::loop_scope_shape::LoopScopeShape;
-use crate::mir::BasicBlockId;
 use std::collections::HashSet;
 
 /// Extract all variable names from an AST expression