[flutter_markdown] Remove ignored file (#351)

Fixes a new publish warning by removing a file that should be ignored,
but was checked in.

Part of https://github.com/flutter/flutter/issues/82368
diff --git a/packages/flutter_markdown/flutter_markdown.iml b/packages/flutter_markdown/flutter_markdown.iml
deleted file mode 100644
index 4f01810..0000000
--- a/packages/flutter_markdown/flutter_markdown.iml
+++ /dev/null
@@ -1,17 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<module type="WEB_MODULE" version="4">
-  <component name="NewModuleRootManager" inherit-compiler-output="true">
-    <exclude-output />
-    <content url="file://$MODULE_DIR$">
-      <excludeFolder url="file://$MODULE_DIR$/.pub" />
-      <excludeFolder url="file://$MODULE_DIR$/build" />
-      <excludeFolder url="file://$MODULE_DIR$/example/packages" />
-      <excludeFolder url="file://$MODULE_DIR$/packages" />
-      <excludeFolder url="file://$MODULE_DIR$/test/packages" />
-    </content>
-    <orderEntry type="inheritedJdk" />
-    <orderEntry type="sourceFolder" forTests="false" />
-    <orderEntry type="library" name="Dart SDK" level="project" />
-    <orderEntry type="library" name="Dart Packages" level="project" />
-  </component>
-</module>
\ No newline at end of file