Remove gpu replace directive

The gpu module is now published at github.com/luxfi/gpu.
go mod tidy removed the gpu dependency as it was orphaned (not actually used).

go.mod

@@ -3,7 +3,6 @@ module github.com/luxfi/fhe
 go 1.25.5
 
 require (
-	github.com/luxfi/gpu v0.29.4
 	github.com/luxfi/lattice/v7 v7.0.0
 	github.com/redis/go-redis/v9 v9.7.0
 	github.com/stretchr/testify v1.10.0
@@ -21,7 +20,3 @@ require (
 	golang.org/x/sys v0.33.0 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )
-
-// GPU bindings require local C++ compilation
-// TODO: Rename github.com/luxfi/mlx to github.com/luxfi/gpu to fix module path
-replace github.com/luxfi/gpu => /Users/z/work/luxcpp/gpu