test: drop unused _clamp import from math.jl

`_clamp` was never defined in the source — only `_dead_zone` exists.
The Blocks/math.jl test imported both via `using
ModelingToolkitStandardLibrary.Blocks: _clamp, _dead_zone`, but neither
is used in this file (`_dead_zone` is only used in
Blocks/nonlinear.jl, which has its own import). Removing the entire
`using` line fixes a UndefVarError that errored out all 93 tests in
math.jl.

Verified locally: 93/93 pass after the fix.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

Author: ChrisRackauckas

test/Blocks/math.jl

@@ -1,7 +1,6 @@
 using ModelingToolkitStandardLibrary.Blocks
 using SciCompDSL
 using ModelingToolkit, OrdinaryDiffEq, Test
-using ModelingToolkitStandardLibrary.Blocks: _clamp, _dead_zone
 using ModelingToolkit: inputs, unbound_inputs, bound_inputs, t_nounits as t
 using OrdinaryDiffEq: ReturnCode.Success
 using OrdinaryDiffEqRosenbrock: Rodas4