fix: tweak spacing in send asset picker

[BZahory]

Description

The token symbol send page's asset picker is not vertically aligned with other dropdowns on the page. This PR removes an unneeded margin introduced in #25470.

Related issues

Fixes:

Manual testing steps

1. Go to send page
2. Ensure asset picker is spaced correctly and vertically aligned with other components

Screenshots/Recordings

Before

After

Pre-merge author checklist

• [x] I've followed MetaMask Contributor Docs and MetaMask Extension Coding Standards.
• [x] I've completed the PR template to the best of my ability
• [x] I’ve included tests if applicable
• [x] I’ve documented my code using JSDoc format if applicable
• [x] I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• [ ] I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• [ ] I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.