gn2bp: use discard instead of remove

discard succeeds even if entry does not exist. This is in preparation of
moving architecture dependent sources inside an arch object.

Test: //components/cronet/android:cronet
Change-Id: I71646bf031da864d0dfd07c04ebfe275f72c2fbe
diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index 1e6913e..2ab61cd 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -1237,7 +1237,7 @@
   # need arch condition. So, for now, hardcoding arch condition
   if module.name == "cronet_aml_base_allocator_partition_allocator_partition_alloc":
     x86_srcs = module.srcs.copy()
-    x86_srcs.remove(
+    x86_srcs.discard(
       "base/allocator/partition_allocator/starscan/stack/asm/x64/push_registers_asm.cc")
     x86_srcs.add("base/allocator/partition_allocator/starscan/stack/asm/x86/push_registers_asm.cc")
     module.arch['x86'] = {'srcs': x86_srcs}