Revert "Allow passing config as lazy-evaluated function"

This reverts commit 1819cf67d3.

src/index.js

@@ -13,10 +13,6 @@ import substituteBreakpointAtRules from './lib/substituteBreakpointAtRules'
 import substituteClassApplyAtRules from './lib/substituteClassApplyAtRules'
 
 const plugin = postcss.plugin('tailwind', (options = {}) => {
-  if (_.isFunction(options)) {
-    options = options()
-  }
-
   const config = mergeConfig(defaultConfig, options)
 
   return postcss([