Clean up the old Desktop Incognito NTP

1. Remove the code of the old ("non-MD") Incognito NTP
2. Remove the qualifier "MD" from variable names (no longer necessary,
   as the "non-MD" version is removed)
3. Remove the qualifier "MD" from file names (as above; and thus
   overwrite the previous files).

Note: #3 was executed as:
> mv md_incognito_tab.css incognito_tab.css
> mv md_incognito_tabsjs incognito_tab.js
> mv md_incognito_tabshtml incognito_tab.html
And then inside md_incognito_tabs.html:
> s/md_incognito_tab/incognito_tab/
Although git understood this as delete+rename, the diffs in the code
compare the old files with the new ones, so it's not as well visible :(

Bug: 693525
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I8e4096337dff097aaf400f15b2c2b7b8e1c9c475
Reviewed-on: https://chromium-review.googlesource.com/777319
Commit-Queue: Martin Šrámek <[email protected]>
Reviewed-by: Evan Stade <[email protected]>
Reviewed-by: Michael Giuffrida <[email protected]>
Cr-Commit-Position: refs/heads/master@{#520584}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index e27aeba..4db93de 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2397,7 +2397,7 @@
      FEATURE_VALUE_TYPE(features::kMaterialDesignBookmarks)},
     {"enable-md-incognito-ntp",
      flag_descriptions::kMaterialDesignIncognitoNTPName,
-     flag_descriptions::kMaterialDesignIncognitoNTPDescription, kOsAll,
+     flag_descriptions::kMaterialDesignIncognitoNTPDescription, kOsAndroid,
      FEATURE_VALUE_TYPE(features::kMaterialDesignIncognitoNTP)},
     {"safe-search-url-reporting",
      flag_descriptions::kSafeSearchUrlReportingName,