Remove unnecessary curl installation

[devdrops]

At https://github.com/devdrops/php-toolbox/blob/master/Dockerfile#L17-L19, we have the following:

RUN apt-get update > /dev/null 2>&1 && \
    apt-get -y install \
        curl \

And so on. BUT, at https://github.com/devdrops/php-toolbox/blob/master/Dockerfile#L14, we have the following:

RUN curl --silent https://getcomposer.org/installer | php > /dev/null 2>&1 && \

This is a call to curl, which points that curl is already installed.