uucore: format: Fix default Float precision in try_from_spec

The default precision is 6, no matter the format. This applies
to all float formats, not just "%g" (aka FloatVariant::Shortest).

Fixes #7361.

src/uucore/src/lib/features/format/num_format.rs

@@ -293,13 +293,7 @@ impl Formatter for Float {
 
         let precision = match precision {
             Some(CanAsterisk::Fixed(x)) => x,
-            None => {
-                if matches!(variant, FloatVariant::Shortest) {
-                    6
-                } else {
-                    0
-                }
-            }
+            None => 6, // Default float precision (C standard)
             Some(CanAsterisk::Asterisk) => return Err(FormatError::WrongSpecType),
         };