Remove double.fromEnvironment from dart-define doc (flutter#124102)

Remove double.fromEnvironment from dart-define doc.

Fixes flutter#124665 

The current dart-define docs reads:

```
Additional key-value pairs that will be available as constants from the String.fromEnvironment, bool.fromEnvironment, int.fromEnvironment, and
double.fromEnvironment constructors.
```
while dart-define-from-file reads:

```
The path of a json format file where flutter define a global constant pool. 
Json entry will be available as constants from the String.fromEnvironment, bool.fromEnvironment, int.fromEnvironment, 
and double.fromEnvironment constructors; the key and field are json values
```

<img width="1440" alt="Screenshot 2023-04-03 at 19 31 34" src="https://user-images.githubusercontent.com/31275429/229720102-2020e1a9-c1fb-470b-89e6-ffb84096a51b.png">

However, there is no fromEnvironment constructor for double. The aim of this PR is to address this by removing double.fromEnvironment from help docs as stated in

Author: Mastersam07

packages/flutter_tools/lib/src/runner/flutter_command.dart

@@ -603,8 +603,8 @@ abstract class FlutterCommand extends Command<void> {
       FlutterOptions.kDartDefinesOption,
       aliases: <String>[ kDartDefines ], // supported for historical reasons
       help: 'Additional key-value pairs that will be available as constants '
-            'from the String.fromEnvironment, bool.fromEnvironment, int.fromEnvironment, '
-            'and double.fromEnvironment constructors.\n'
+            'from the String.fromEnvironment, bool.fromEnvironment, and int.fromEnvironment '
+            'constructors.\n'
             'Multiple defines can be passed by repeating "--${FlutterOptions.kDartDefinesOption}" multiple times.',
       valueHelp: 'foo=bar',
       splitCommas: false,
@@ -617,7 +617,7 @@ abstract class FlutterCommand extends Command<void> {
       FlutterOptions.kDartDefineFromFileOption,
       help: 'The path of a json format file where flutter define a global constant pool. '
           'Json entry will be available as constants from the String.fromEnvironment, bool.fromEnvironment, '
-          'int.fromEnvironment, and double.fromEnvironment constructors; the key and field are json values.\n'
+          'and int.fromEnvironment constructors; the key and field are json values.\n'
           'Multiple defines can be passed by repeating "--${FlutterOptions.kDartDefineFromFileOption}" multiple times.',
       valueHelp: 'use-define-config.json',
       splitCommas: false,