fix: The HTTP Header of `multipart/form-data` no longer includes `charset`.

[dongfangtianyu]

Description

close #6250 .

Motivation and Context

On some web server implementations, including charset in the request header Content-Type of multipart/form-data can result in parsing errors of the boundary, leading to a failure in sending form content.

This is inconsistent with the behavior in JMeter 5.6.2 and also does not comply with RFC.

Perhaps fixing this in the httpclient would be a more elegant choice, but:

• The last update on httpclient 4.x was 2 years ago, so it would be slow.
• This issue occurred in JMeter 5.6.3 (#5987). This PR will revert JMeter's behavior to 5.6.2, reducing user frustration.

How Has This Been Tested?

1. A unittest case
2. runGUI then e2e test

Screenshots (if appropriate):

After ： remove charset

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code follows the code style of this project.
• [ ] I have updated the documentation accordingly.