Remove usb option from manual printer configuration.

chrome/browser/resources/settings/printing_page/cups_add_printer_dialog.html

 <link rel="import" href="chrome://resources/html/md_select_css.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/web_ui_listener_behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-input/paper-input.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-spinner/paper-spinner.html">
 <link rel="import" href="../i18n_setup.html">
 <link rel="import" href="cups_add_printer_dialog_util.html">
 <link rel="import" href="cups_printers_browser_proxy.html">
 <link rel="import" href="../settings_shared_css.html">
@@ skipping 155 matching lines @@
                 <select class="md-select" 
                     value="[[newPrinter.printerProtocol]]"
                     on-change="onProtocolChange_">
                   <option value="ipp">$i18n{printerProtocolIpp}</option>
                   <option value="ipps">$i18n{printerProtocolIpps}</option>
                   <option value="http">$i18n{printerProtocolHttp}</option>
                   <option value="https">$i18n{printerProtocolHttps}</option>
                   <option value="socket">$i18n{printerProtocolAppSocket}
                   </option>
                   <option value="lpd">$i18n{printerProtocolLpd}</option>
-                  <option value="usb">$i18n{printerProtocolUsb}</option>

[xdai1, 2017/03/17 03:39:49]

drive-by comment: I think you should also remove
IDS_SETTINGS_PRINTING_CUPS_PRINTER_PROTOCOL_USB and the corresponding string
from settings_strings.grdp?

[stevenjb, 2017/03/17 16:55:11]

That would be correct.

[skau, 2017/03/17 19:52:40]

No, it's still used on the printer detail page. 
https://cs.chromium.org/chromium/src/chrome/browser/resources/settings/printi...

We can still setup USB printers.  It's just that setting up USB printers in this
flow doesn't make a lot of sense anymore.

                 </select>
                 <span class="md-select-underline"></span>
               </div>
             </div>
           </div>
         </div>
         <div class="settings-box two-line">
           <div class="start">
             <div class="label">$i18n{printerQueue}</div>
             <div class="secondary">
@@ skipping 183 matching lines @@
     <!-- Manufacturer and Model Dialog -->
     <template is="dom-if" if="[[showManufacturerDialog_]]" restamp>
       <add-printer-manufacturer-model-dialog new-printer="{{newPrinter}}"
           setup-failed="[[setupFailed]]">
       </add-printer-manufacturer-model-dialog>
     </template>
 
   </template>
   <script src="cups_add_printer_dialog.js"></script>
 </dom-module>