debian/control: remove unnecessary versioned dependency on pkg-php-tools

Remove an unnecessary versioned dependency on pkg-php-tools, as
suggested by the following cme warning:

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Warning in 'control source Build-Depends:1': unnecessary greater-than versioned dependency: pkg-php-tools (>= 1.7~). Debian has oldoldstable -> 1.28; oldstable -> 1.35; stable -> 1.37; unstable -> 1.37; testing -> 1.37;
Offending value: 'pkg-php-tools (>= 1.7~)'
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