[CASHTAB] Remove the no longer needed --legacy-peer-deps install flag

Summary:
As per title.

Depends on D8833.

Test Plan:
  npm install

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8834

web/cashtab/Dockerfile

@@ -14,7 +14,7 @@ RUN apt-get update && \
   apt-get install -y git && \
   apt-get install -y python
 # install node modules and build assets
-RUN npm install --legacy-peer-deps && npm run build
+RUN npm install && npm run build
 
 # Stage 2
 FROM nginx

web/cashtab/README.md

@@ -9,10 +9,8 @@
 
 ## Development
 
-CashTab relies on some modules that retain legacy dependencies. NPM version 7 or later no longer supports automatic resolution of these peer dependencies. To successfully install modules such as `qrcode.react`, with NPM > 7, run `npm install` with the flag `--legacy-peer-deps`
-
 ```
-npm install --legacy-peer-deps
+npm install
 npm start
 ```
 
@@ -67,7 +65,7 @@ See the section about [deployment](https://facebook.github.io/create-react-app/d
 ## Docker deployment
 
 ```
-npm install --legacy-peer-deps
+npm install
 docker-compose build
 docker-compose up
 ```