fix: don't leave out non-prefixed routes for generate + prefix strategy

[rchl]

To avoid having to create server-side redirect rules, the non-prefixed
routes are not removed when using prefix strategy with Nuxt generate.
The non-prefixed routes should automatically redirect to default locale
routes.

Resolves #700

[codecov]

Codecov Report

Merging #726 into master will decrease coverage by 1.56%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #726      +/-   ##
==========================================
- Coverage   99.16%   97.60%   -1.57%     
==========================================
  Files           3        3              
  Lines         120      125       +5     
  Branches       33       34       +1     
==========================================
+ Hits          119      122       +3     
- Misses          1        3       +2     

Impacted Files	Coverage Δ
src/helpers/routes.js	96.61% <50.00%> (-3.39%)	⬇️
src/index.js	97.82% <100.00%> (+0.15%)	⬆️

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