fix: drop stale _clamp import (un-break Blocks/math tests)

test/Blocks/math.jl imported `_clamp` from Blocks, but `_clamp` was
removed from the source — the Limiter component now calls the builtin
`clamp` directly (src/Blocks/nonlinear.jl). The dead import made the
2nd Core testset error with `UndefVarError: _clamp` on every Julia/dep
combination, so the whole suite (and downgrade CI) fails at master.
`_clamp` is unused in the test; `_dead_zone` (still in source) is kept.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

test/Blocks/math.jl

@@ -1,7 +1,7 @@
 using ModelingToolkitStandardLibrary.Blocks
 using SciCompDSL
 using ModelingToolkit, OrdinaryDiffEq, Test
-using ModelingToolkitStandardLibrary.Blocks: _clamp, _dead_zone
+using ModelingToolkitStandardLibrary.Blocks: _dead_zone
 using ModelingToolkit: inputs, unbound_inputs, bound_inputs, t_nounits as t
 using OrdinaryDiffEq: ReturnCode.Success
 using OrdinaryDiffEqRosenbrock: Rodas4