Bug 1801894 - `modules/fdlibm`: remove dead file. r=sylvestre

Differential Revision: https://phabricator.services.mozilla.com/D163313

modules/fdlibm/src/moz.build

@@ -56,7 +56,7 @@ SOURCES += [
     's_cbrt.cpp',
     's_ceil.cpp',
     's_ceilf.cpp',
-    's_copysign.cpp',
+    # 's_copysign.cpp', # Unused file.
     's_cos.cpp',
     's_expm1.cpp',
     's_fabs.cpp',