Remove double.fromEnvironment from dart-define doc.

Fixes #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