remove build concurrency workaround

- no longer needed
- didn't actually fix the actual issue which is because of rollup-plugin-dts not vitepress build
- also buildConcurrency shouldn't be decided on the basis of number of threads but the available memory, it is used to compute how many pages are rendered at once, all of that is run on a single thread

docs/.vitepress/config.ts

@@ -6,15 +6,6 @@ import {
 } from 'vitepress-plugin-group-icons'
 import llmstxt from 'vitepress-plugin-llms'
 
-let buildConcurrency: number
-try {
-  const { cpus } = await import('os')
-  buildConcurrency = 4 * cpus().length
-} catch {
-  // edge services may not expose os module
-  buildConcurrency = 16
-}
-
 export default defineConfig({
   title: 'VitePress',
 
@@ -25,7 +16,6 @@ export default defineConfig({
   lastUpdated: true,
   cleanUrls: true,
   metaChunk: true,
-  buildConcurrency,
 
   markdown: {
     math: true,