Fix conflicts in IAR configuration

In `targets.json`, the base target ARM_MPS2_Target does not have `iar`
in `supported_c_libs`. But its inherited targets have `IAR` in
`supported_toolchains`, causing configuration check to fail.

Author: LDong-Arm

targets/targets.json

@@ -4423,8 +4423,7 @@
         "core": "Cortex-M0",
         "supported_toolchains": [
             "ARM",
-            "GCC_ARM",
-            "IAR"
+            "GCC_ARM"
         ],
         "extra_labels": [
             "ARM_SSG",
@@ -4465,8 +4464,7 @@
         "core": "Cortex-M0+",
         "supported_toolchains": [
             "ARM",
-            "GCC_ARM",
-            "IAR"
+            "GCC_ARM"
         ],
         "extra_labels": [
             "ARM_SSG",
@@ -4505,8 +4503,7 @@
         "core": "Cortex-M3",
         "supported_toolchains": [
             "ARM",
-            "GCC_ARM",
-            "IAR"
+            "GCC_ARM"
         ],
         "extra_labels": [
             "ARM_SSG",
@@ -4546,8 +4543,7 @@
         "core": "Cortex-M4F",
         "supported_toolchains": [
             "ARM",
-            "GCC_ARM",
-            "IAR"
+            "GCC_ARM"
         ],
         "extra_labels": [
             "ARM_SSG",
@@ -4587,8 +4583,7 @@
         "core": "Cortex-M7FD",
         "supported_toolchains": [
             "ARM",
-            "GCC_ARM",
-            "IAR"
+            "GCC_ARM"
         ],
         "extra_labels": [
             "ARM_SSG",