remove "Renew Keys" button.

it wasn't doing anything.
closes https://github.com/lnbits/lnbits/issues/147

lnbits/core/templates/core/wallet.html

@@ -219,9 +219,6 @@
   <div class="col-12 col-md-5 q-gutter-y-md">
     <q-card>
       <q-card-section>
-        <q-btn flat color="grey" @click="exportCSV" class="float-right"
-          >Renew keys</q-btn
-        >
         <h6 class="text-subtitle1 q-mt-none q-mb-sm">LNbits wallet</h6>
         <strong>Wallet name: </strong><em>{{ wallet.name }}</em><br />
         <p></p>