Fix(input): remove nspin=4 guard on gint_precision to enable mixed-precision

The gint mixed-precision path (mix/single) was artificially blocked
for nspin=4, but the DeltaSpin subspace code already supports fp32
GEMM with proper cast_down/cast_up. Remove the guard.

Test (LCAO nspin=4 DeltaSpin subspace gga_grad=3):
- double precision: Etot = -6267.46519 eV, Stress_xx = -1103.396 kbar
- mix precision:    Etot = -6267.46517 eV, Stress_xx = -1103.396 kbar
- Difference: ~2e-5 eV, stress identical

Author: dyzheng

source/source_io/module_parameter/read_input_item_system.cpp

@@ -763,12 +763,7 @@ Available options are:
                     "ReadInput",
                     "gint_precision = single or mix is currently supported only for LCAO calculations.\n");
             }
-            if (para.inp.gint_precision != "double" && para.inp.nspin == 4)
-            {
-                ModuleBase::WARNING_QUIT(
-                    "ReadInput",
-                    "gint_precision = single or mix is not supported for nspin = 4 (noncollinear/SOC) calculations.\n");
-            }
+
         };
         this->add_item(item);
     }