chore: remove deprecated macosX64 target (#625)

Remove `macosX64()` target from multiplatform configuration

## Motivation and Context
`macosX64` is [deprecated in Kotlin
2.2](https://kotlinlang.org/docs/whatsnew2220.html#deprecation-of-x86-64-apple-targets)
and scheduled for removal in
[2.3](https://kotlinlang.org/docs/native-target-support.html#deprecated-targets)

## How Has This Been Tested?
—

## Breaking Changes
for users of macosX64, no longer get a native `macosX64` artifact

## Types of changes
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

buildSrc/src/main/kotlin/mcp.multiplatform.gradle.kts

@@ -52,7 +52,6 @@ kotlin {
             }
         }
     }
-    macosX64()
     macosArm64()
     linuxX64()
     linuxArm64()