refactor: remove SSG exports from bundles

[lucacasonato]

getStaticData and getStaticPaths are now not included in the client bundles anymore.

main:

Page                           Size     First Load JS
┌ ● /                          232 B    5.95 kB      
├ ○ /dynamic/:name             175 B    5.89 kB      
└ ● /user/:name                286 B    6 kB         
                                                     
+ First Load JS shared by all  5.72 kB               
  ├ /main-e304fa6e.js          2.22 kB               
  └ /preact-5e05c32a.js        3.5 kB                

○  (Static)  automatically rendered as static HTML
●  (SSG)     automatically generated as static HTML + JSON (uses getStaticData)

this PR:

Page                           Size     First Load JS
┌ ● /                          189 B    5.9 kB       
├ ○ /dynamic/:name             175 B    5.89 kB      
└ ● /user/:name                189 B    5.9 kB       
                                                     
+ First Load JS shared by all  5.71 kB               
  ├ /main-3437414c.js          2.22 kB               
  └ /preact-5e05c32a.js        3.5 kB                

○  (Static)  automatically rendered as static HTML
●  (SSG)     automatically generated as static HTML + JSON (uses getStaticData)

File sizes are measured after brotli compression.