refactor(bash/bashrc): remove a conditional statement from `nvm-upgrade` alias
refactor(bash/bashrc): remove a conditional statement from `nvm-upgrade` alias

This alias should be declared only if (1) the `nvm` command exists and
(2) the `$NVM_DIR` directory exists. The `if` block is executed only
when the opposite of these two conditions is met; thus, enclosing the
alias with an extra if conditional statement is redundant.

shoichiaizawa/dotfiles,shoichiaizawa/dotfiles,shoichiaizawa/dotfiles