#4215 Multilingual: Remove language code in alternate link 

This PR to solve the issue described in https://github.com/joomla/joomla-cms/issues/4215 when language is the default site language and remove url language code is on.

Example of result (en-GB is default site and url language code is on:
Home page before patch:
```
  <link href="http://localhost:8888/testwindows/trunkgitnew/en" rel="alternate" hreflang="en-GB" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/de/" rel="alternate" hreflang="de-DE" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/it/" rel="alternate" hreflang="it-IT" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/es/" rel="alternate" hreflang="es-ES" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/mk/" rel="alternate" hreflang="mk-MK" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/ta/" rel="alternate" hreflang="ta-IN" />
```
Home page after:
```
  <link href="http://localhost:8888/testwindows/trunkgitnew/" rel="alternate" hreflang="en-GB" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/de/" rel="alternate" hreflang="de-DE" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/it/" rel="alternate" hreflang="it-IT" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/es/" rel="alternate" hreflang="es-ES" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/mk/" rel="alternate" hreflang="mk-MK" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/ta/" rel="alternate" hreflang="ta-IN" />
```
Another associated item after patch:
```
 <link href="/testwindows/trunkgitnew/fr/instructions-multilangue.html" rel="canonical" />
  <link href="/testwindows/trunkgitnew/fr/instructions-multilangue.feed?type=rss" rel="alternate" type="application/rss+xml" title="RSS 2.0" />
  <link href="/testwindows/trunkgitnew/fr/instructions-multilangue.feed?type=atom" rel="alternate" type="application/atom+xml" title="Atom 1.0" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/multi-lingual-steps-by-steps.html" rel="alternate" hreflang="en-GB" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/de/schritt-für-schritt-zur-mehrsprachigkeit.html" rel="alternate" hreflang="de-DE" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/it/multilingua-passo-dopo-passo.html" rel="alternate" hreflang="it-IT" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/es/multiidioma-paso-a-paso.html" rel="alternate" hreflang="es-ES" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/mk/повеќејазичност-чекор-по-чекор.html" rel="alternate" hreflang="mk-MK" />
  <link href="http://localhost:8888/testwindows/trunkgitnew/ta/பன்-மொழி-படிப்படியாக.html" rel="alternate" hreflang="ta-IN" />
```