[FIX] code generator: prevent AST change

This commit removes an AST change during the code generation of slots.
Before, the `compileTSlot` function deleted the `t-props` attribute
directly on `ast.attrs`. This creates wrong code when compiling a
second time as the `t-props` attribute does not exist anymore.

tests/compiler/__snapshots__/t_slot.test.ts.snap

@@ -0,0 +1,13 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`t-slot compile t-props correctly multiple time 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { callSlot } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    return callSlot(ctx, node, key, 'default', false, Object.assign({}, {a:1}));
+  }
+}"
+`;