Remove unused code from Lexeme frontend data model

Remove unused features of the Lexeme:

• language
• lexical category
• equals()
• isEmpty()

Note: does not remove statements which are actually used
depends on browser test to check this

Depends-On: Ic7b3abb0df2bea1b79c9fabde5af600115eb9d52

Bug: T198379
Change-Id: I2c08b81edafaf0edc1a4c15384fc8683317f2afe