Remove unused dolinsolve import from linear_solver_split_ode_tests

The import was a leftover from the original file and causes an error
on Julia LTS (1.10) since dolinsolve is not exported from OrdinaryDiffEqCore.

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

Author: ChrisRackauckas

lib/OrdinaryDiffEqNonlinearSolve/test/linear_solver_split_ode_tests.jl

@@ -1,7 +1,6 @@
 using Test
 using OrdinaryDiffEqBDF, OrdinaryDiffEqSDIRK
 using LinearAlgebra, LinearSolve
-using OrdinaryDiffEqCore: dolinsolve
 
 n = 8
 dt = 1 / 1000