[OpenMP][deviceRTLs] Remove omp_is_initial_device

`omp_is_initial_device` in device code was implemented as a builtin
function in D38968 for a better performance. Therefore there is no chance that
this function will be called to `deviceRTLs`. As we're moving to build `deviceRTLs`
with OpenMP compiler, this function can lead to a compilation error. This patch
just simply removes it.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D95397

GitOrigin-RevId: 3333244d77c44e8bb5af57027646596f7714ff62
diff --git a/libomptarget/deviceRTLs/common/src/libcall.cu b/libomptarget/deviceRTLs/common/src/libcall.cu
index f43d74a..c1cfe3e 100644
--- a/libomptarget/deviceRTLs/common/src/libcall.cu
+++ b/libomptarget/deviceRTLs/common/src/libcall.cu
@@ -316,11 +316,6 @@
   return rc;
 }
 
-EXTERN int omp_is_initial_device(void) {
-  PRINT0(LD_IO, "call omp_is_initial_device() returns 0\n");
-  return 0; // 0 by def on device
-}
-
 // Unspecified on the device.
 EXTERN int omp_get_initial_device(void) {
   PRINT0(LD_IO, "call omp_get_initial_device() returns 0\n");
diff --git a/libomptarget/deviceRTLs/interface.h b/libomptarget/deviceRTLs/interface.h
index 5f539bc..e27e4fa 100644
--- a/libomptarget/deviceRTLs/interface.h
+++ b/libomptarget/deviceRTLs/interface.h
@@ -89,7 +89,6 @@
 EXTERN int omp_get_num_devices(void);
 EXTERN int omp_get_num_teams(void);
 EXTERN int omp_get_team_num(void);
-EXTERN int omp_is_initial_device(void);
 EXTERN int omp_get_initial_device(void);
 EXTERN int omp_get_max_task_priority(void);